The present invention relates to a pneumatic tire having an improved tread portion which is capable of preventing the wandering phenomenon of a vehicle when running on a road provided with rain grooves. The present invention is also directed to a tire tread which is capable of reducing the running noise of the tire.
In recent years, as the expressway network is developed and vehicle performance is remarkably improved, to obtain a superior drainage performance during high speed running, tires having wide main grooves extending substantially straight in the circumferential direction of the tire are widely used, for example, in passenger cars.
In some regions in some countries, to improve drainage and thereby to prevent accidental slip under wet conditions, so called rain groove are provided on the roads surface. The rain grooves are parallel narrow grooves extending in the longitudinal direction of the road and their pitch in the transverse direction of the road are about 15 to 30 mm.
Therefore, when tires having the above-mentioned pattern run on such a road, the edges of the tire grooves and the edges of the rain grooves often engaged with each other, and the rain groves interfere with the lateral movement of the tire. This is felt by the driver as a wandering phenomenon of the vehicle. If the pitch of the main grooves of a tire coincides with the pitch of the rain grooves, there is the possibility that the tire grooves and the rain grooves will tightly engaged each other during running, which can be very dangerous.
In a tread pattern including straight wide main grooves, a so called air resonance noise is generated when running on well-paved roads at high speed. The air existing in the tubes formed between the road surface and main grooves in the ground contacting patch is excited by vibrations of the tread elements and pulsative air flow, and the air resonates at a certain frequency (about 800 to 1250 Hz) to generate an air resonance noise. If the depth of the main grooves is decreased, the resonance noise can be reduced, but the wet performance of the tire is greatly decreased.
An object of the present invention is therefore to provide a pneumatic tire in which the wandering phenomenon caused by rain grooves is effectively prevented.
Another object of the present invention is to provide a pneumatic tire in which the running noise due to air resonance in the main grooves is effectively reduced without sacrificing the wet performance of the tire.